1. Field of the Invention
The invention relates to a magnetic sensor as well as an apparatus and a method for checking documents of value. The document of value can be bank notes, identity documents, check cards, credit cards, share certificates, deeds, stamps, checks, admission tickets, tickets, flight tickets, identification documents, visa stickers, labels, seals, packagings or other such elements. The simplifying term “document of value” in the following therefore always includes documents of the mentioned type.
2. Related Art
From the prior art it is known to provide documents of value with security elements, such as security strips or also security threads, which contain magnetic material. The magnetic material here can be applied to the security element either continuously or only in certain areas, for example in the form of a coding. For the magnetic coding of a security element there is used for example a certain sequence of magnetic and nonmagnetic areas, which is characteristic of the type of the document of value to be secured. Furthermore, it is known to use different magnetic materials for a magnetic coding, for example with different coercive field strengths. In the hitherto known magnetic codings there are used for example two differently coercive magnetic materials, from which there are formed two types of magnetic areas which can be arranged side by side or one above the other.
Furthermore, it is known to automatically check bank notes with security threads, which have a magnetic coding of differently coercive materials. Here, the bank notes are transported parallel to the course of the security element, and one after the other first go through a strong magnetic field parallel to the transport direction, which magnetizes both the high-coercive and also the low-coercive magnetic areas along the transport direction. The remaining magnetization is checked by means of an inductive magnetic head which is sensitive exclusively parallel to the transport direction. Then the bank notes go through a weaker magnetic field perpendicular to the transport direction, which orients only the low-coercive magnetic areas perpendicular to the transport direction, while the high-coercive magnetic areas remain magnetized in the transport direction. Again, the remaining magnetization is checked by means of an inductive magnetic head which is sensitive exclusively in parallel to the transport direction. Here, with the first inductive magnetic head there are detected the high- and the low-coercive magnetic areas and with the second inductive magnetic head there are detected only the high-coercive magnetic areas. If, however, differently coercive magnetic areas reach the detection area of the magnetic sensor at the same time, said magnetic sensor detects a superposition of the magnetic signals of the differently coercive magnetic areas. A disadvantage of this method is that magnetic areas which contain both high- and low-coercive magnetic material are difficult to distinguish from individual high-coercive or low-coercive magnetic areas.